Integral hydraulic power steering gears are commonly used in trucks, heavy equipment such as earth-moving vehicles, and construction vehicles. “Integral” refers to a steering gear containing a manual steering mechanism, a hydraulic control valve assembly, and a hydraulic power cylinder integrated into a single unit.
The hydraulic power cylinder typically comprises a chamber divided into two chamber portions by a piston. The piston has a set of teeth which mesh with a sector gear fixed to an output shaft. The output shaft is connected via steering linkage to steerable wheels of a vehicle to steer the vehicle when the output shaft is rotated.
The hydraulic control valve assembly controls the flow of pressurized hydraulic fluid between a hydraulic pump and one of the chamber portions to control the direction and amount of steering. The valve assembly typically comprises two relatively rotatable valve elements, one of which is connected to a rotatable input shaft operatively coupled to the vehicle steering wheel. The other valve element is connected with a follow-up member, such as a ball screw drive, which rotates in response to movement of the piston. The ball screw drive provides a direct connection between the input shaft and the piston to allow for manual steering of the vehicle in the event of hydraulic fluid pressure loss.
In the typical integral hydraulic power steering gear, the input shaft is connected to the vehicle steering wheel by one or more intermediate shafts. The intermediate shafts are usually long and can be prone to excessive lash. The required routing of the intermediate shaft also restricts the placement of the steering gear components and other components on the vehicle. It is desirable to eliminate the intermediate shaft from the vehicle steering system. If the intermediate shaft is eliminated, there is no mechanical connection between the steering wheel and the steering gear. Such systems are known, and are commonly referred to as “steer-by-wire” systems.
Because there is no mechanical connection between the steering wheel and the steering gear, redundancy in steer-by-wire systems may be desirable for fail-safe reasons.
It is also known to use an electrically powered steering apparatus to turn the steerable wheels instead of a hydraulically powered steering gear. One known electric steering apparatus for turning the steerable vehicle wheels includes a ball nut for transmitting force between an electric motor and an axially movable member. Upon actuation of the electric motor, the ball nut is driven to rotate relative to the member. The rotational force of the ball nut is transmitted to the member by balls to drive the member axially. Axial movement of the member effects turning movement of the steerable wheels.
In another known electric power steering apparatus, an electric motor is connected with gearing which provides a gear reduction between an electric motor shaft and an output pinion meshed with an axially movable member. Rotation of the output pinion by the electric motor causes the member to move axially to turn the steerable wheels.